The field of the disclosure relates generally to power plants and, more particularly, to a control system for managing steam turbine rotor stress and methods of use thereof.
A typical known combined cycle power plant includes a gas turbine in which a fuel, such as, for example, and without limitation, oil, or natural gas, is burned. Exhaust gasses from the gas turbine engine are pushed out by one or more gas turbines. The exhaust gasses pass over a heat recovery steam generator, heating the water within to generate steam. The gas turbine load and exhaust gas temperature are highly coupled due to combustion system and control limitations. Such limitations generally result in a one-to-one relationship between gas turbine exhaust mass flow rate and gas turbine exhaust temperature, which lead to a direct one-to-one relationship between steam flow rate and steam temperature. The steam is collected in a boiler and builds up pressure. When a sufficient steam pressure is reached, the steam is admitted into the steam turbine, thereby rotating the steam turbine. The steam is then typically condensed and returned to the heat recovery steam generator. The steam turbine rotates a generator to generate electricity. The steam admittance mechanism to the steam turbine can be configured in multiple configurations, including, for example, and without limitation, architectures where steam is heated and reheated in multiple stages and admitted to the steam turbine in multiple stages.
When not operating for a period of time, the shell and rotor of the steam turbine cool significantly. Temperature of the steam turbine rotor may be gauged by a temperature measured on the steam turbine rotor itself or on the shell, i.e., the fixed portion, rather than on the rotor, i.e., the rotatable portion. Alternatively, steam turbine rotor temperature may be gauged by any other suitable method, including, for example, and without limitation, non-contact methods such as pyrometry. During startup of a combined cycle power plant, the plant is ideally brought to conditions where plant load can be controlled without constraints as quickly as possible. However, the surface of the steam turbine rotor typically heats at a different rate than the bulk of the rotor, resulting in radial temperature variations throughout the rotor. Such temperature variations manifest as thermal stress on the metal of the rotor and contribute to rotor metal fatigue over time. Because the temperature of the gas turbine exhaust gases drive steam temperature, the overall plant is constrained by the need to keep gas turbine exhaust flow at a lower temperature until the steam turbine rotor has been brought to conditions where rotor surface stresses are not limiting.